The data provided by ten DONET deep-sea observatories, that on March 11, 2011, registered the Great East Japan Earthquake and tsunami, were used for investigation of the relationship between variations of the ocean bottom pressure and three-component accelerograms. Methods of cross-spectral analysis revealed the existence of a frequency range of "forced oscillations, " within which pressure variations are proportional to the vertical component of the acceleration. This proportionality is manifested by the magnitude-squared coherence (MSC) being close to unity and a phase lag (PL) practically equal to zero. The spectral analysis method showed the proportionality coefficient to be equal the mass of a water column of unit area at the installation point of the observatory or, approximately, to the product of the water density and the ocean depth. The observed boundaries of the frequency range of "forced oscillations" are revealed to correspond to the theoretical frequency values confining the manifestation of surface gravity and acoustic waves in pressure variations near the ocean bottom. The hypothesis is put forward that the small deviations of MSC from unity and of PL from zero observed by a number of stations within the range of "forced oscillations" are due to the contribution of horizontal movements of nearby submarine slopes. A theoretical analysis has been performed of the problem of forced oscillations of a water layer in a basin of varying depth. A formula is obtained that relates pressure variations at the ocean bottom to acceleration components of the bottom motion and the bottom slope. The pressure in the region of forced oscillations is shown to decrease exponentially with the distance from the moving segment of the ocean bed, so pressure variations, originating from movements of the bottom, are registered effectively by a gauge at the ocean bottom only within a radius less than 1-2 ocean depths. A cross-spectral analysis of pressure variations and of three-component accelerograms confirmed the hypothesis concerning the contribution of horizontal movements of nearby submarine slopes to pressure variations. which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

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The Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET) is a submarine cabled real-time seafloor observatory network for precise earthquake and tsunami monitoring. Ten DONET observatories were in operation during the 2011 Tohoku-Oki event near the Pacific coast of Honshu Island. Each observatory was equipped with an ocean bottom pressure gauge (PG) and a threecomponent ocean bottom seismometer (OBS). A comparative analysis of the PG and OBS records revealed that shortly after seismic surface waves traversed the DONET region, free gravity waves were observed within the water layer. The period of these gravity waves was approximately 170 s, the peak-to-peak amplitude was approximately 3.5 cm, the length was on the order of 22 km, and the phase velocity was 134 m/s. We performed numerical simulations of the observed gravity waves using a combined 2D/3D numerical model. The ground motions required for the simulation were reconstructed from records provided by the DONET OBSs and the nearest ground-based GPS stations. The synthetic bottom pressure variations are in good agreement with the DONET PG records. The synthetic displacements of the ocean surface throughout the simulation domain showed that the observed gravity waves were excited directly above the submarine slopes. Theoretical estimates and numerical experiments revealed the generation mechanism of the observed gravity waves.Plain Language Summary During submarine earthquakes, the ocean floor shifts from its original position. Due to this displacement, a huge volume of water is moved, which can generate a destructive tsunami wave. However, earthquakes are accompanied not only by displacement of the ocean bottom in the epicentral area but also by the emission of seismic waves. These seismic waves are also capable of exciting waves in the ocean. For our study of this phenomenon, we used unique data from the Japanese Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET) observatories that were recorded during the Great East Japan Earthquake on 11 March 2011. Based on the records of the DONET seismometers, we reconstructed the movement of the bottom during the passage of a surface seismic wave. Then, using the obtained information, we simulated the movement of the water in the entire DONET area. The results of our simulation are in good agreement with the records of the DONET pressure gauges. The results also reveal that waves are only generated directly above steep submarine slopes during the passage of seismic waves. This study can help to identify tsunami wave signals in the records of bottom pressure gauges.

An algorithm is presented for testing the calibration accuracy of both z-accelerometers and pressure gauges (PG) installed in seafloor observatories. The test is based on the linear relationship between the vertical acceleration component of the seafloor movement and variations of the seafloor pressure, which is a direct consequence of Newton's 2-nd law and holds valid in the frequency range of “forced oscillations.” The operability of the algorithm is demonstrated using signals registered by 28 observatories of the DONET-2 system during 4 earthquakes of magnitude Mw ~ 8 that took place in 2018-2019 at epicentral distances from 55° up to 140°.